With the advancement of portable and cordless electronic instruments, secondary batteries that are smaller in size, lighter in weight, higher in energy density and contain nickel hydrogen and lithium ions have received attention as a power supply for driving them.
Regarding a nickel hydrogen secondary battery, in order to increase the energy density, the porosity of spongy metal is increased by increasing the active material density or thickness of the electrode plate. Especially, in a cylindrical secondary battery, the electrode plate is wound, and hence the electrode plate cracks or breaks in a winding process.
An example is disclosed where the active material density on the outer peripheral side of the electrode plate of a cylindrical battery is made lower than that on the inner peripheral side (for example, patent document 1). Here, the cylindrical battery has an active material support body made of strip spongy metal and an active material layer formed in the active material support body. According to patent document 1, since the active material density on the outer peripheral side is lower, the flexibility during winding is increased and crack or breakage hardly occurs.
Regarding a lithium ion secondary battery, a non-aqueous electrolyte secondary battery is presently in actual use where the negative electrode current collector employs a negative electrode having a negative electrode mixture layer including a negative electrode active material made of graphite material. Generally, the negative electrode is manufactured by coating metal foil as the negative electrode current collector with negative electrode mixture paste, which contains a negative electrode active material, conductive agent, and binder, and by drying them. The negative electrode after drying is often increased in density by rolling to adjust its thickness to a predetermined thickness. The secondary battery is produced by winding the negative electrode and a positive electrode via a separator. However, the theoretical capacity density of the secondary battery is 372 mAh/g (833 mAh/cm3), and further increase in energy density is required.
Recently, as the negative electrode active material whose theoretical capacity density exceeds 833 mAh/cm3, silicon (Si), tin (Sn), germanium (Ge), an oxide thereof, and an alloy thereof which can form alloys with lithium have been studied. In particular, silicon-containing particles such as silicon particles and silicon oxide particles have been widely studied because they are less expensive.
However, the volumes of these materials increase when they insert lithium ions. For instance, when a negative electrode having an active material made of Si is used, the negative electrode active material in the maximally inserted state of lithium ions is expressed by Li4.4Si. The volume increase rate is 4.12 when the state changes from Si to Li4.4Si.
Insertion and extraction of lithium ions cause expansion and contraction of the negative electrode active material. During repeating the charge-discharge cycle, reduction in adhesiveness of the negative electrode active material to the negative electrode current collector can cause peeling or the like.
For addressing such a problem, an example is disclosed where a thin film of a negative electrode active material having an inverse pyramidal structure having a gap is formed near the surface of a negative electrode current collector having the irregular surface, expansion stress is reduced, and the current collecting property is secured (for example, patent document 2).
However, the secondary battery of patent document 1 cannot provide a predetermined capacity, because the active material densities on the outer peripheral side and the inner peripheral side of an electrode plate of the battery are different from each other. Furthermore, compression stress occurring on the inner peripheral side in winding makes the amount of supply electrolyte on the inner peripheral side smaller than that of electrolyte supplied to the active material on the outer peripheral side, so that the capacity further decreases disadvantageously.
In a secondary battery having a negative electrode active material made of graphite, also, expansion and contraction of about 1.2 times are caused by charge and discharge. Especially, in an electrode group formed by winding a negative electrode and a positive electrode via a separator, a negative electrode mixture layer on the inner peripheral side of a negative electrode current collector receives compression stress, and a negative electrode mixture layer on the outer peripheral side receives tensile stress. When strain stress due to the expansion and contraction of the negative electrode active material during the charge and discharge is applied to this state, strain occurs in the negative electrode mixture layer. As a result, a conductive network in the negative electrode mixture layer crashes, the negative electrode mixture layer peels from the negative electrode current collector, the facing state of the positive electrode to the negative electrode is made non-uniform, and hence the cycle characteristic degrades disadvantageously.
However, generally, the strain of the negative electrode mixture layer is reduced by a binder or conductive agent existing around the negative electrode active material, so that the cycle characteristic does not sharply degrade.
Since the negative electrode mixture layer is compressed on the inner peripheral side of the negative electrode current collector and is stretched on the outer peripheral side by winding, the density of the negative electrode active material facing the positive electrode mixture layer varies between the inner peripheral side and the outer peripheral side. Thus, the insertion and extraction amount of lithium ions varies between facing the positive electrode mixture layer and negative electrode mixture layer, so that the lithium ions cannot be effectively used disadvantageously.
Due to the compression on the inner peripheral side of the negative electrode current collector and the stretch on the outer peripheral side by winding, the porosity of the negative electrode mixture layer varies between the inner peripheral side and the outer peripheral side. Therefore, the difference between the amounts of non-aqueous electrolytes containing the lithium ions restricts the battery capacity disadvantageously.
In the non-aqueous electrolyte secondary battery of patent document 2, also, the density and porosity of the negative electrode active material vary by winding between the inner peripheral side and the outer peripheral side of the negative electrode current collector. Therefore, the battery capacity achievable in a planar structure without winding cannot be obtained disadvantageously. Especially, in the negative electrode active material such as silicon-containing particles whose expansion-contraction ratio is large, larger strain occurs on the inner peripheral side. Therefore, only a non-aqueous electrolyte secondary battery having extremely low cycle characteristic and reliability can be achieved.    [Patent document 1] Japanese Patent Unexamined Publication No. H06-76819    [Patent document 2] Japanese Patent Unexamined Publication No. 2002-313319